The atom is the smallest unit of matter and is composed of protons, neutrons, and electrons. Atoms have a small but dense nucleus at the center, containing protons and neutrons, as discovered by Ernest Rutherford in his 1909 gold foil experiment. Various models were proposed to explain the structure of the atom, starting with Democritus' idea of basic elements made of atoms. John Dalton introduced the idea that elements are made of unique atoms, while J.J. Thomson's "plum pudding" model had a positive sphere with electrons embedded in it. Rutherford determined atoms had a small, dense nucleus through his gold foil experiment, and Niels Bohr added the concept of electron energy levels to explain atomic stability

1. ATOM Thesmallest unitofmatteras
recognizedby chemicalpropertiesof
molecules.
composed ofprotons,neutronsandelectrons.
very small; typicalsizesarearound100
picometers(aten-billionthof ameter,inthe
shortscale)
Nucleus isthesmall,denseregion
consistingofprotonsandneutronsat the
centerof anatom,discoveredin1911by
ErnestRutherfordbasedonthe1909
Geiger–Marsden goldfoil
experiment.

2. Radioactivity
Thespontaneousdecayof atomicnuclei.
Duringradioactivity,alpha particles,beta-
rays andgammaraysareemitted.
DEMOCRITUS
■Therearevariousbasicelementsfrom
whichallmatterismade■Everythingis
composedof smallatomsmovinginavoid
■Someatomsareround,pointy,oily,have
hooks, etc.toaccountfortheirproperties■
Ideasrejectedbyleadingphilosophers
becausevoid= noexistence

3. BILLIARDBALLconceptoftheatom,
whereintheatomisan incrediblysmallbut
smoothandwhole object
ROBERTBROWN
Heisknownfor hisideaoftherandom
movementofmicroscopicparticlesor
"Brownianmotion."
JOHNDALTON
■Introducedhisideasin1803thateach
elementiscomposedof extremelysmall
particlescalledatoms
Proponent:JohnDaltonThismodel
suggestedthatatoms:• arethesmallest
particleof anelement•of differentelements

4. havedifferentmasses• aresolid,
indestructibleunits.
J.J.THOMSON
Conceptualizedfollowingcathoderay
experiments
Firstmodeltouseideaof subatomicparticles
Thomson “PlumPudding”Model
ActualPlumPudding
J.J.THOMSON
ERNESTRUTHERFORD
Hasanucleus
Conceptualizedfollowingα-particle
experiments

5. GoldFoilExperiment
NIELSBOHR
Hasanucleus
Hasenergylevelsorquanta
Explainswhyelectronsdon’tfallintothe
nucleus
IntheBohrmodeloftheatom,electrons
travelindefinedcircularorbitsaroundthe
nucleus.
Theorbitsarelabeledbyan integer,the
quantumnumbern.
Electronscanjumpfromoneorbittoanother
byemittingorabsorbingenergy.
SchrödingerHeisenberg

6. QUANTUMMODEL
Hasanucleus
Hasenergylevelsorquanta
Explainswhyelectronsdon’tfallintothe
nucleus
Hasideaof orbitals
Thismodelusescomplexshapesoforbitals
(sometimescalledelectronclouds),volumes
ofspaceinwhichthereislikelytobean
electron.
JAMESCHADWICK
JamesChadwickwasastudentof
Rutherford’swhobuilton thispossibilityin
1926.Heworkedon radiationemittedby

7. berylliumthattooktheform ofparticles
heavyenoughtodisplaceprotons.
GoldFoilExperiment